Your general understanding is correct, I just have some comments.
In case of synthetic full the merge of 7 files occurs once a week, while forever forward merges 1 file during each run.
The main advantage of SF is when you are keeping restore points for more than a week you end up with multiple VBKs for potentially faster restores and cleaner VIB management.
I would not say that`s an advantage. With forever forward you always have only 1 full backup, so it requires less space.
Note that in forever forward incremental mode, during the backup process itself there is only 1 I/O to the repository, a single write, which means the VM backup will complete much faster, the other 2 I/Os are performed "post-process" during a merge at the end of the job.
Here is a good relative